The present invention relates to automated welders, and more particularly to programmable automated welders. More particularly still, the invention relates to a novel user-programmable resistance welder especially of the "series-weld" type.
In resistance welding, "spot" welds are produced by coalescence of the workpiece itself into weld "nuggets" which are produced by the heat obtained from the resistance offered by the workpiece to the flow of electric current in a circuit of which the workpiece is a part, as a function of the application of pressure to the workpiece through the welding electrodes during current flow.
Robotic resistance welders have heretofore been developed which automatically repetitively weld identical workpieces. However, such welders have had limited versatility and capability, and have only been capable of making "direct" type of resistance welds, in which a pair of axially aligned mutually spaced welding electrodes receive the workpiece between them and the electrodes are then moved toward one another into contact with opposite sides of the workpiece in order to make a weld.
"Direct" resistance welders have many disadvantages, however, including comparatively high current requirements and resultant high power consumption due to length of weld loop on large panels, the inherent production of unsightly sunken recess areas ("sinks") where each electrode contacts the workpiece (i.e., on both sides of the workpiece), and the requirement of substantial open space on both sides of the workpiece to accommodate the two opposed and mutually aligned electrodes of each set or pair thereof. Because of this alignment requirement, the aligned electrodes are usually fixed in place and the workpiece is moved from point-to-point relative to the workpiece in order to produce welds at the different desired places on the workpiece. Precise control and coordination of the two electrodes located on opposite sides of the workpiece has been extremely difficult, and robotic welders typically have very limited control capabilities, most being designed for a single application and requiring complete mechanical reconfiguration to operate on each different workpiece.
Series-type resistance welding has several characteristics which are preferable to those associated with direct resistance welding, in particular substantially lower power requirements and current flow, and as a result exponentially lower power loss. Additionally, in series-type resistance welding both of the electrodes in a pair or set are located on the same side of the workpiece. This eliminates the difficulties imposed by the requirement of having open and unobstructed areas throughout a substantial volume of space on both sides of the workpiece in which to move the welding.electrodes from point to point over the surface of the workpiece in completing the welding schedule, as is true in direct welding. At the same time, however, the degree of control required for series-type welding is far more demanding and complex than is true for direct resistance welding, since the path for current flow between the electrodes traverses the entire length of the stock between a given pair of weld points, whereas in direct welding the path merely involves current flow through the thickness of the stock. Consequently, since any two given weld points may be located at randomly varying distances from one another, which often involves changes in stock thickness as well, the parameters of current flow and electrode force ("squeeze") will typically change for each ensuing pair of welds, and of course each individual weld in a pair may themselves have different parameters. Of course, as is known to those skilled in the welding art, any given resistance weld may involve a succession of different current flow and squeeze characteristics while the electrode remains generally in position at the weld location, such changes typically being timed by use of line current cycles as the basic time interval, i.e., in varying multiples of one-sixtieth second each. Consequently, the weld command for each given location is likely to be complex, and in the case of series-type resistance welding the added degrees of complexity have, it is believed, heretofore precluded even conceiving of an automated, programmable series-type resistance welder such as the present invention provides.